The present invention relates to a method of treating waste effluent, in particular photographic effluent, which contain reduced species, especially sulphur-oxygen species, such as, for example, thiosulphate or sulphite. The present invention also embraces apparatus for performing such methods of treatment.
It is usual for photographic effluent for both black-and-white and color processing to contain one or more reduced sulphur-oxygen species. Thiosulphate or xe2x80x9chypoxe2x80x9d is commonly used as a fixing agent, and sulphite ions are often included in developer solutions and/or stop/clearing baths as a preservative scavenger for oxidized developing agent. Sulphite is also known as a fixing agent DE-A-3635219 to AGFA-GEVAERT AG discloses treating dilute aqueous solutions of thiosulphate by oxidation with hydrogen peroxide in the presence of a chromium, vanadium, tungsten or molybdenum compound as catalyst and discloses that molybdate is preferred. It is desirable to oxidize thiosulphate in photographic effluent before discharging the effluents to a sewage system, in order to reduce the chemical oxygen demand (COD) of the effluent.
A problem with the treatment method disclosed by DE-A-3635219 is that, while the COD of the thiosulphate-containing photographic effluent is reduced, transition metal contaminants are added to the effluent as catalyst which are subsequently discharged into the environment. This is clearly undesirable for environmental reasons, and is also an inefficient use of the transition metals. It is an object of the present invention to provide an improved treatment method for waste effluent, and in particular photographic effluent, containing reduced species, which does not involve the discharge of transition metal species to the environment and in which the transition metal species can be reused.
According to one aspect of the present invention therefore there is provided a method of treating waste effluent containing reduced species, by oxidation with hydrogen peroxide, or a compound capable of releasing hydrogen peroxide, in the presence of a catalyst therefor, characterized in that said catalyst is immobilized on a substrate therefor.
The reduced species may generally be a sulphur-oxygen species, typically as thiosulphate or sulphite.
Said catalyst may be selected from molybdate, tungstate, chromate and vanadate, although tungstate and especially molybdate are preferred.
Typically the substrate will constitute a porous mass which permits permeation of the photographic effluent into its interstices, thereby presenting a large surface area of catalyst to the effluent. Said substrate may be anionic, and in a particular aspect of the present invention the substrate comprises an anion exchange material. In some embodiments, a mixture of anion and cation exchange materials may be used. The material may typically be a polymeric resin, especially a polyacrylic or polystyrene resin, or clay or zeolite-type material.
In another aspect, the present invention contemplates a photographic development process in which effluent from one or more of the development steps are treated continuously or batch-wise by the method according to the present invention. Usually, the effluent from each stage will be combined and treated together. Said development process may be a redox-amplification (RX) process which is performed upon a photographic element containing a reduced silver lay-down density as compared with conventional silver halide photographic materials. As will be well known to a person skilled in the art, hydrogen peroxide is employed in the development step of an RX process, in which case it may be unnecessary to add additional peroxide to the photographic effluent. Of course, additional peroxide may be added to the effluent if required.
The above method is therefore particularly appropriate for use with dilute solutions containing fixer, for example those with less than about 20 g thiosulphate (based on ammonium thiosulphate), and hence is suitable for RX developed images.
However it has been found that with those fixer solutions wherein the amount of sulphur-oxygen species is high, i.e. conventional solutions such as those used in x-ray and graphic arts, and wherein sufficient peroxide is added to effect total oxidation to sulphate, the pH may become very acid (even as low as pH 1) which would be corrosive and unsuitable for discharge directly to sewers. The effluent could be subsequently neutralized by adding an alkali or base to the treated liquid in an amount appropriate to render the pH suitable for disposal, generally around pH 5-9, but this would require a separate process. Adding an alkali metal hydroxide or carbonate having a high pH to the added peroxide solution however causes the peroxide to decompose rapidly and therefore it cannot be stored therewith.
In accordance with a further aspect of the invention the above problem has been overcome by the addition to the peroxide solution of a soluble alkali whose conjugate acid has a pKa of  less than 8.5, such as for example a soluble bicarbonate, alkanoate, e.g. acetate, or dihydrogen phosphate, in an amount approximately equivalent to the acid released in the decomposition reaction. Only very slight decomposition of peroxide over a period of several weeks results.
The soluble alkali is an alkali metal or ammonium salt, preferably a bicarbonate and more preferably the highly soluble potassium bicarbonate since the solubility of the alkali limits the amount of the acid that can be neutralized. At low pH the bicarbonate is converted to carbon dioxide which escapes from the solution thereby not adding to the salt content, which would be the case for other salts except for hydroxide or carbonate. Potassium bicarbonate is also the least polluting alkali, as it has no biological oxygen demand (BOD5), and is also the cheapest, making it particularly suitable for practice in the present invention.
The pH of the effluent is raised by the above process but cannot exceed the natural pH of the alkali (about pH 9) so if there is a pump fault anywhere in the apparatus then the pH of the effluent can never go too high. Moreover provided an excess of alkali is added the actual amount required is not critical.
Conveniently the peroxide/alkali mixture may be stored as a combined oxidation/neutralizing solution which can then be pumped through a single pump and thence together with the fixer solution over the immobilized substrate as described hereinafter.
Compounds capable of releasing hydrogen peroxide include metal peroxides; compounds which include hydrogen peroxide in their crystal structure such as sodium percarbonate; other peroxy compounds such as sodium perborate and persulphate; or soluble organic peroxide, such as butyl peroxide or benzyl peroxide. The peroxide is added in an amount sufficient to cause oxidation of a substantial proportion of the reduced species and is conveniently hydrogen peroxide itself.
Where the photographic effluent is treated batch-wise, the catalyst may be disposed within a receptacle which is equipped with an inlet for introducing photographic effluent from the development process and an outlet for discharging treated effluent to waste. The outlet will be fitted with selectively operable closing means for closing the outlet during conduction of the treatment method, typically a valve.
Alternatively, the treatment method may be performed continuously on effluent delivered from the photographic development process. In another aspect of the present invention therefore the catalyst/substrate may be packed in a conduit which is arranged to receive photographic effluent in one end, and to deliver the treated photographic effluent from the other end. The flow rate of photographic effluent through the conduit will be adjusted such that the average residence time of effluent within the conduit is sufficient to oxidize a substantial proportion of the reduced sulphur-oxygen species contained in the effluent
In the method wherein alkali is added to the peroxide in storage, the experimental set-up is as described hereinafter.
Where thiosulphate-containing effluents from the fixing stage are treated in accordance with the present invention, silver that has been complexed during fixing may be precipitated in the treatment stage. In some embodiments therefore silver-bearing precipitate may be separated from the treated photographic effluent before the effluent is discharged. For this, filtering or centrifuging means may be employed.
According to the present invention therefore a method of treating waste effluent containing reduced species, preferably containing sulphur-oxygen species, is provided which does not involve discharging transition metal species into a sewage system. This is advantageous from the environmental perspective, and also means that the transition metal catalysts can be reused which is more efficient in these materials as compared with the prior art processes. Unexpectedly, the present applicants have found that by supporting the transition metal catalysts on a substrate therefor, substantially less catalyst can be used as compared with the prior art processes without impairing the efficiency of the treatment reaction. For example, the treatment method of the present invention can be performed effectively using less than 1/100 parts by weight catalyst, and typically less than 1/250 parts by weight of sulphur-oxygen species. It was also found surprisingly that use of a supported catalyst in accordance with the present invention appeared to cause or allow more complete destruction of sulphur-oxygen species to sulphate as compared with prior art processes using unsupported catalyst.
A method is further provided wherein addition of alkali, such as potassium bicarbonate, to the peroxide solution in the case of effluents with high fixer concentration enables the pH of the treated effluent to be maintained at environmentally acceptable levels.